(i) Field of the Invention
The present invention relates to shielding structures for a high frequency circuit for microwave or millimeter wave band, particularly to shielding structures and methods for a high frequency circuit to obtain both functions of electromagnetic shielding and hermetic seal by using a complex adhesive.
(ii) Description of the Related Art
A conventional shielding structure for a high frequency circuit for microwave or millimeter wave band will be described with reference to FIGS. 7A and 7B. FIGS. 7A and 7B illustrate a shielding structure in case of using a microwave integrated circuit (to be referred to as MIC hereinafter) as an example of high frequency circuit.
In these drawings, FIG. 7A shows a plan view of the MIC 10 and FIG. 7B shows a sectional view of the MIC 10 along the line Axe2x80x94A.
In FIGS. 7A and 7B, the reference numeral 1 denotes a metal base plate (to be referred to as metal stem hereinafter). The reference numeral 2 denotes a dielectric substrate which is mounted on the metal stem 1 and on which a high frequency circuit (e.g., a semiconductor chip for a high frequency amplifier, micro-strip lines, etc.) is formed. The reference numeral 3 denotes a metal cap covering the whole of the dielectric substrate 2 to serve for high frequency shielding. The reference numeral 4 denotes a conductive adhesive such as silver paste filling in the joint between the metal cap 3 and the metal stem 1 to bond them and to serve for electric shielding for the high frequency circuit. The reference numeral 5 denotes a hermetically sealing adhesive or welding applied to the whole outer periphery of the joint between the metal cap 3 and the metal stem 1 to serve for hermetic seal for the MIC. The reference numeral 6 denotes interface connection terminals for an external circuit, which terminals are hermetically sealed with the metal stem 1 and connected to the high frequency circuit of the dielectric substrate 2.
The conventional shielding structure for the high frequency circuit as described above has need of sufficient electric shielding because the high frequency circuit for microwave band is placed within the metal cap 3. Besides, it has need of sufficient hermetic shielding for preventing external moisture or the like from entering the interior of the MIC.
For the electric shielding condition, the conductive adhesive 4 is applied to the joint between the metal cap 3 and the metal stem 1 to ensure electric shielding. Besides, in order to meet the hermetic shielding condition, in addition to the above conductive adhesive 4, the hermetically sealing adhesive or welding 5 covers the whole periphery to obtain hermetic seal.
The conventional shielding structure for the high frequency circuit as described above has need of the hermetically sealing adhesive or welding 5 for hermetic shielding after applying the conductive adhesive 4 for electric shielding. So, shielding must be made through two stages of manufacturing processes, i.e., an electric shielding process and a hermetic shielding process.
For this reason, the conventional shielding structure for the high frequency circuit requires a complex working process and much time for manufacture.
It is an object of the present invention to provide shielding structure and method for a high frequency circuit with a simple manufacturing process and a great shielding effect.
A shielding structure for a high frequency circuit according to the present invention comprises:
a metal stem;
a high frequency circuit mounted on an upper portion of the metal stem;
a metal cap for covering the periphery of the high frequency circuit from above; and
a complex adhesive in which a ring-shaped electromagnetic shielding adhesive and a ring-shaped hermetically sealing adhesive are joined by thermo-compression bonding, and which is provided at the joint between the metal cap and the metal stem.